This invention relates to a process for the production of crystalline propene-ethene-butene-1 terpolymers with up to 10% by weight of ethene and up to 5% by weight of butene-1 in accordance with the low-pressure method with the aid of mixed catalysts of crystalline TiCl.sub.3 and/or TiCl.sub.3 . n AlCl.sub.3 (n = 0.2 to 0.6), on the one hand, with chlorine-containing organoaluminum compounds, on the other hand, optionally in the presence of hydrogen.
In accordance with a process not pertaining to the state of the art (German Patent Application P 23 38 478.3 corresponding to copending, commonly assigned U.S. Pat. Application Ser. No. 491,172, filed July 23, 1974), an isotactic polypropylene and/or a crystalline copolymer thereof with up to 10% butene-1 is obtained by polymerization of propene in butene-2 or in a butene-2-containing C.sub.4 -cut which can include up to 5% of butene-1. The polymerization in butene-2 or a C.sub.4 -cut containing butene-2 has the advantage that the propylene can also be polymerized at moderate pressures with the use of evaporative cooling, and that the thus-obtained polypropylene can be processed by means of a simple operation.
The polymers produced according to this process have a high isotactic proportion and accordingly show high yield points with comparably good notch impact strength values. It is desirable for various fields of application to still further increase the notch impact strength values of these polymers.
It is known that the impact strength toughness of polypropylene can be increased by the copolymerization of propene with ethene. This is generally accomplished by a stepwise polymerization. According to this method, one monomer, e.g. the propene, is first polymerized. After the polymerization of this monomer, e.g. the propene, the unreacted propene monomer is adiabatically expanded, optionally thereafter purged with an inert gas, and only then is the second monomer, preferably ethene, added and polymerized. This stepwise polymerization of propene and ethene can optionally be repeated several times for the production of block copolymers. By polymerization in several stages, the formation of amorphous propene-ethene copolymers which impede processing and lower the strength values is prevented. However, this procedure has the great disadvantage that it is very complicated and expensive due to the required removal of residual monomer. Furthermore, larger amounts of ethene are required in this procedure to improve the notch impact strength, namely about 15 - 30%, based on the propene.
Consequently, there is interest in a process making it possible to produce a polypropylene having an improved impact strength by a simple operation during both polymerization and subsequent processing.